1. Field of the Invention
The present invention relates to methods for treating papermaking press felts and reducing or eliminating the need for batch cleaning. More specifically the invention relates to the continuous or intermittent treatment of press felts with enzymes, alone or in combination with felt conditioning chemistries to inhibit deposition or filling on or within the felt structure.
2. Discussion and Background
Paper is produced in a continuous manner from a fibrous suspension (pulp furnish) generally made of water and cellulose fibers. A typical paper manufacturing process consists of 3 stages: forming, pressing, and drying. In the forming stage, dilute pulp furnish is directed on a wire or between 2 wires. The majority of the water is drained from the pulp furnish, through the wire, creating a wet paper web. In the pressing stage the paper web comes in contact with one or generally more porous press felts that are used to extract much of the remaining water from the web. Often the pickup felt is the first felt that the wet paper web contacts which is used to remove the paper web from the wire, via a suction pickup roll positioned behind the felt, and then to transport the paper web to the rest of the press section. The paper web then generally passes through one or more presses each consisting of rotating press rolls and/or stationary elements such as press shoes that are positioned in close proximity to each other forming, what is commonly referred to as, a press nip. In each nip the paper web comes in contact with either one or two press felts where water is forced from the paper web and into the press felt via pressure and/or vacuum. In single-felted press nips the paper web is in contact with the press roll on one side and the felt on the other. In double-felted press nips, the paper web passes between the two felts. After the press section, the paper web is dried to remove the remaining water, usually by weaving through a series of steam heated dryer cans.
Press felts often consist of nylon base fabric generally made of from 1 to 4 individual layers of filaments arranged in a weave pattern. An extruded polymeric membrane or mesh can also be included as one or more of the base fabric layers. Batt fibers, of smaller diameter than the base fabric filaments, are needled into the base on both sides giving the felt a thick, blanket-like appearance. Press felts are designed to quickly take in water from the paper web in the nip and hold the water so that it does not re-absorb back into the sheet as the paper and felt exit the press nip. Press felts are normally an endless loop that circulates continuously in a belt-like fashion between sheet contact stages and return stages. Water pulled into the felt from the paper web at the nip is generally removed from the felt by vacuum during the felt return stage at, what is frequently referred to as, the uhle box.
A variety of materials can be dissolved or suspended in the liquid contained in the paper web when it reaches the press felt and these materials can therefore be transferred into the press felt along with the water extracted from the paper web. Unfortunately some of these materials tend to stay with the press felt and accumulate there instead of being removed with the water at the uhle box. Some of the dissolved or suspended materials that are present in the paper web and can deposit in the felt include components originating from the fibrous pulp such as cellulose fines, hemicelluloses, and sticky components such as wood pitch from fresh wood pulps and glues, resins, and waxes from recycled pulps. Byproducts of microbiological growth such as polysaccharides, proteins, and other biological matter, can also be present in the stock and therefore in the press felts. Various functional additives that are added to paper stock to impart certain properties to the finished paper can also find their way to the press felts. These additives include sizes such as rosin, alkyl ketene dimer (AKD), and alkenyl succinic anhydride (ASA); wet strength resins and dry strength agents for example starch; and inorganic fillers including clay, talc, precipitated or ground calcium carbonate (PCC, GCC), and titanium dioxide. Processing additives used to improve or limit problems during paper production that can also end up in press felts include retention and drainage aids including alum, organic polymers, and various micro-particles; and defoamers, in particular those based on oil.
It is important for efficient paper production, that press felts remain deposit-free. Deposits that form on press felts such as oily or sticky materials can transfer back to the web resulting in dirt spots or holes in the finished paper. They can also cause paper breaks or tears leading to lost production. It is also important for efficient paper production, that press felts remain porous with high void volume. It is highly expensive and energy intensive to evaporate water from paper in the dryer section, making it critical that the press felts remove as much water as possible from the paper web in the press section. Felts that become filled with contaminants that limit water movement through the felt will thus limit the amount of water that can be removed from the web. This will force the machine speed to be slowed in order to allow time for the web to dry in the dryer section. Felts that are unevenly filled can also lead to uneven water removal from the sheet which can result in moisture streaks, wrinkles, and web breaks.
Some hydrophobic materials such as waxes can form a barrier layer at the felt surface preventing water from entering the felt. Other hydrophobic materials, that are tacky or sticky, such as pitch and defoamer oils can increase felt compaction, causing a loss in void volume, thus limiting the amount of water that can enter the press felt. Deposits containing particulate materials on or embedded within the press felt structure can result in significant wear problems limiting the life of the press felt. PCC is particularly problematic, due to its sharp edges and rigid surface that can damage, cut, and prematurely wear out the felt fibers. Some hydrophilic materials such as, starches, proteins, and hemicelluloses tend to exist within the felt in the form of gels that can actually trap water, as well as other depositing materials, within the felt thus limiting the amount of water that can be removed at the uhle box. These hydrophilic gels are particularly problematic in felts since currently used felt conditioning treatments are ineffective at inhibiting them.
It is well known in the art that felt conditioners enhance the performance and extend the effective life of felts by minimizing formation of certain deposits. Felt conditioners are usually liquid blends of surfactants, dispersants and/or polymers most often in water but other solvents are also utilized. Oxidizers, acids, and alkalis can also be contained in felt conditioners, generally in relatively low concentrations. Felt conditioners are applied continuously or intermittently to papermaking felts while paper is being produced through showers during the fabric return stage, while the felt is not in contact with the paper web. These treatments are most often applied on the inside, or machine side, of the felt through low pressure showers, often just prior to a felt carrier roll such that hydraulic force will help move the chemical into the felt to help prevent and remove contaminants that fill the felt. Such treatments are also sometimes applied, through similar showers on the sheet side of the felt after the uhle box and before the nip so that the treatment is present on the surface when contaminants first reach the felt. Additional water showers that are commonly used on press felts and where chemicals could be used include high pressure showers that are usually employed intermittently, so as not to damage the felt, and are most often used on the sheet side to remove surface contaminants. Lubrication showers are also commonly used to apply water at the entrance to the uhle box to prevent wear and provide a seal so that vacuum can remove fluid from within the felt; if desired a chemical treatment could be included within this shower.
When the felts become too filled that they no longer allow for efficient paper manufacture, it becomes necessary to clean them by a process commonly referred to as batch cleaning. When felts are batch cleaned, paper production is stopped, the felt speed is generally slowed, the vacuum at the uhle box is stopped or significantly reduced, and showers are turned off with the exception of the chemical shower. A cleaning solution, generally consisting of high concentrations of caustic, acid, solvent such as kerosene, and/or oxidizer such as hypochlorite, is applied through the chemical shower. After sufficient time for the cleaning solutions to penetrate the filling material, water showers are employed such that the contaminants and batch cleaning chemicals are removed from the felt by vacuum at the uhle box. It is generally necessary to remove the batch cleaning chemicals from the press felt because these materials, at the high concentrations utilized, can damage the press felt if allowed to remain on the felt or can transfer back to the paper altering its characteristics. In some instances it may be necessary to batch clean felts multiple times in a 24-hour production day. Batch cleaning is often necessary, but not a desirable solution since the chemicals used are often hazardous, environmentally unfriendly, and can damage the felt with repeated use. Valuable production time is lost during shut downs for batch cleaning. If such cleaning is unsuccessful, it is necessary to remove the felt, sometimes prematurely, from the paper machine, which is costly from both a time and material perspective.
Continuous and intermittent felt conditioners have been successful at reducing felt filling and increasing time between batch cleanings. However there are still materials that fill felts that are not effectively inhibited by felt conditioning treatments. In particular, existing felt conditioners have limited impact on hydrophilic contaminants such as starch, hemicellulose, and proteinaceous materials which tend to form hydrogels within press felts limiting water movement through the felt and trapping other contaminants. By providing improved felt conditioning methods the frequency of batch cleaning will be reduced. Current felt conditioning practices dictate that a relatively high level of surfactant and/or dispersant must be disposed of since felt conditioners are applied continuously. If sewered, these materials can lead to environmental problems of aquatic toxicity and/or biodegradability. If water from the uhle box containing the conditioners is recycled back into the white water system, surfactants and dispersants are known to lead to problems in paper production such as losses in paper sizing.
It has long been believed that the use of enzymes for felt conditioning was impractical or impossible due to the long reaction times assumed to be required. The general consensus of specialty chemical manufacturers quoted in Tappi Journal Survival Techniques: Extending the Life of Press Fabrics (July 1997, Vol. 80, No. 7, p. 58) was that the residence time within the fabric was not long enough for enzymes to react with the substrate to achieve significant degradation of the problematic material. The only potentially practical application noted was for use as batch cleaners for washing felts if the enzymes could be used to replace caustic or acid.
The use of enzymes to batch wash paper making felts during a shut-down when paper is not being produced has been disclosed by WO 97/01669 (Mulder) and U.S. Pat. No. 5,961,735 (Heitmann). Mulder teaches the use of cellulase, xylanase, resinase, amylase, and/or Levan hydrolase sprayed on press felts to remove water binders and bound water. During a shut-down, the felt is first washed with acids and/or bases to remove dissolved materials and then rinsed. Next enzymes are applied and allowed to react on the felt for several minutes followed by a second water rinse. Heitmann teaches a similar procedure where an enzyme solution of cellulase and/or hemicellulase is applied to the felt and allowed to remain there for a period of 1 hour, followed by a rinse with distilled water at 70° C. A solution of sodium hydroxide is then applied to the felt to deactivate the enzyme and the felt is then subjected to a tap water rinse lasting 1 hour. Both methods have the disadvantage of increasing the time necessary to clean felts, during which time valuable production would be lost. They also do not reduce or eliminate the harsh chemistries needed for batch washing since both methods require the use of caustic and/or acids. The paper machine can not be used to produce paper while the felt is being treated by either of these methods.
Heitmann notes that the cleaning method as taught in U.S. Pat. No. 5,961,735 could be employed continuously to press felt while paper was being produced. However, the various contact times, the separate feed of enzyme and then caustic to deactivate the enzyme, and the rinse steps using different types of water would be highly impractical, if not impossible, to employ continuously to a paper machine while it was producing paper.
WO 97/11225 (Pärnänen) discloses the use of enzymes applied to unfelted press rolls to improve paper web release from the press roll as the paper exits the press. The enzymes are applied to the press roll through showers commonly used for lubrication prior to the doctor blade and/or to apply release agents to the roll. The enzymes are claimed to improve paper release by removing a film-like layer of deposition formed on the roll due to substances that originated from the paper web. Pärnänen claims that the invention can be applied to clean other moving elements including paper making wires and felts, however there is no description of how this would be accomplished, no teaching or suggestion whether or not the treatment would be continuously applied or used as a batch cleaner. In the only example used to teach the method for cleaning other moving elements, lypase is shown to enhance the removal of deposits from forming wires by first soaking the wire in enzyme solution then by applying a high-pressure water shower to remove the deposit. In the same example a blend of cellulase and hemicellulase is found to be ineffective. A 24 hour soak in enzyme was used. In contrast, lab examples used to correlate to continuous treatment of the center press roll only required a soak time of 1 hour in more dilute enzyme solutions. This would suggest that Pärnänen's method would require a batch cleaning during a shut down for the other moving parts.
An objective of this invention is to improve the performance of existing felt conditioners with enzymes such that these contaminants are better controlled in order to enhance the effective life of press felts. An additional objective is to provide an alternative approach to traditional felt conditioners such that the use of these chemistries can be reduced or even eliminated with the use of enzymes, which can be deactivated and are completely biodegradable.